This invention is related to digital signal processing, particularly to the estimation of Doppler frequency shift in digital mobile radio communication systems.
Digital mobile radio communication is plagued by distortion in the transmission channel. A significant cause of such distortion is the shift in frequency, i.e., the Doppler shift, caused by the relative motion between a transmitter and receiver. As is known in the art, the Doppler frequency shift may be determined when the relative velocities of the receiving and transmitting systems are known. However, while knowledge of Doppler shift is important to distortion-compensation arrangements (as well as for other purposes), in a typical mobile communication system neither the direction nor the velocity of the mobile unit is known. Hence, the relative velocity between the transmitter and the receiver, and correspondingly the Doppler frequency shift, are unknown quantities.
In digital communications systems, compensation for channel distortion is commonly based on information derived from the insertion of known symbolsxe2x80x94e.g., pilot symbols, at regular intervals in the data stream. When these pilot symbols are recovered at the receiver, any deviation from their known values is taken to be caused by the distortion in the channel. From these deviations, the channel response may be estimated. Various methods of compensating for variations in the channel are known in the art, typically involving a determination of a set of weight factors for an equalizing filter. In general, however, these methods have been found to be of limited use in a dynamically changing signal environment as in a mobile wireless communication systemxe2x80x94where the relative velocity between receiver and transmitter is continually varying. Where one method of compensation may provide acceptable performance under one range of velocity conditions, this same method may not achieve acceptable performance under a different set of such velocity conditions. A distortion compensation methodology was disclosed in the cross-referenced co-pending application, Ser. No. 09/255,088, now abandoned in which compensation weight factors are determined as a function of the Doppler frequency shift between the transmitted and received signal. In this method, if the Doppler frequency can be accurately estimated, then good compensation for the channel distortion may be achieved. However, because the relative velocity between the transmitting and receiving systems is often unknown for wireless mobile systems, an independent means for determining the Doppler frequency shift is required.
The present invention provides an efficient method of determining the Doppler frequency shift that occurs in the reception of a communication signal when the relative velocity between the transmitted and receiver is unknown. By estimating the channel characteristics and measuring the channel response to known transmitted data bits, a measure of the Doppler frequency is determined according to the method of the invention. In particular, a good estimate of the Doppler frequency shift is obtained based on a second order statistic of the fading process for the channel response to the known transmitted data bits. The Doppler frequency shift may then be applied to a process that determines compensation weighting factors to adjust the channel in response to a Doppler shift, as well as for other purposes to which Doppler shift information is needed.